What made you choose the test that you would have to take? I think the best advice is to take a practice test for each and see which you do better at. There are books to check out with practice tests and books you can purchase as well.
You could also just go in to each test and take official ones just to see how you score, but you have to pay for the actual tests. You may do so well in one that you could find this the fastest way to be done with it.
You could also decide based off researching each test. There are four sections in the ACT and three in the SAT. The ACT is described as being more heavily a reading comprehension test. You have a reading, writing, and science section and the science section does not require a lot of outside scientific knowledge, just the ability to read the questions and answer based on what they present on the test. This makes it another reading section, in effect. The SAT just has the traditional Reading, writing, and math. This makes math worth 33% of your grade where on the ACT math is 25% of your grade. This can help kids make the decision as well.
The UC schools, like UCLA, UCI, etc., no longer look at these test scores which makes your GPA and extracurriculars more important. If you are only planning for these schools, I wonder if you will even bother with these tests anymore. If you want to keep options open, many schools either require the scores or list them as optional.
Best of luck as you make decisions on your future.